1. Field of the Invention
The present invention relates to a method for producing silicon monocrystals having a specified defect density. The present invention also relates to a device or apparatus for carrying out the method.
2. The Prior Art
Silicon monocrystals are produced in most practical cases by a crucible pulling method (called Czochralski method or CZ method) or by a crucible-free pulling method (called zone-melting method or FZ method). The rod-shaped monocrystals having diameters of typically 100 to 300 mm serve principally as basic material for producing wafers, from which electronic components are in turn manufactured. On examining the monocrystals and the wafers more closely, defects of varying types and sizes and in varying density (number per spatial or area unit) are found which have been produced during the pulling of the monocrystal.
In connection with the present invention, mention should first be made of the flow pattern defects (hereinafter abbreviated to "FPD"). These can be made visible as etch points having wedge-shaped etching tails after 30 min of Secco etching (H. Yamagishi, I. Fusegawa, N. Fujimaki and M. Katayama, Semicond. Sci. Technol. 7 (1992) A 135). The FPD density directly influences the breakdown strength of an oxide film produced on a silicon wafer. An important, criterion for assessing the breakdown strength is the so-called GOI yield ("gate oxide integrity"). At high FPD densities, the GOI yields are low. Wafers which are provided for producing electronic components ("prime wafers") should have as high GOI yields as possible because the yield of serviceable components also depends on high GOI yields. However, the GOI yield for test wafers ("monitor wafers"), which are used not to produce electronic components but only to check the component production processes, is of rather subordinate importance.
EP-504 837 A2 discloses the fact that dwell times of the growing monocrystal at temperatures of over 1150.degree. C. have a favorable influence on the GOI yield. Anotlier class of defects includes the light point defects (abbreviated to "ILPD" hereinafter), which can be detected on polished wafer surfaces using optical measuring instruments. If light point defects can be detected in high density, the wafers concerned are regarded as inferior both for use as prime welfers and for use as monitor wafers, unless the magnitude of the LPD is predominantly in a range of less than 0.12 .mu.m. In this case, use of the wafers as monitor wafers is quite possible since the defects are not harmful because their size is below the currently standard line width of component structures. In addition, the number of these LPDs can be considerably reduced and the GIO yield increased at the same time by a special heat treatment (annealing), so that wafers treated in this way are also suitable as prime wafers.